The present invention relates to a vacuum treatment system for treating an object, such as a wafer, using a plasma or the like, in a low pressure atmosphere, and to its stage. More particularly, the invention relates to a vacuum treatment system, which is suited to perform etching of a wafer or the like at a high temperature, and to its stage.
Regarding a system for treating an object, such as a wafer, using a plasma or the like, in a low pressure atmosphere, a conventional technique of electrostatically attracting a wafer to a stage using an electrostatic attracting means and treating the wafer with a plasma has been proposed in Japanese Patent Application Publication Nos. 56-53853 and 57-44747. Unexamined Japanese Patent Application Publication Nos. 58-32410 and 60-115226 disclose a wafer treating method including the steps; of electrostatically attracting a wafer to a stage using an electrostatic attracting means, introducing an He gas to the back face of the wafer, and cooling or heating the wafer by thermal conduction, free convection, and forced convection.
In an ordinary etching process, the temperature at the wafer placing face of the stage is controlled to about 80.degree. C. or lower. In some etching processes, however, controlling the wafer temperature to about 100 to 200.degree. C. is needed. More specifically, in the etching of a multilayered film made of a metal and polysilicon, when a metal film is etched, the sample is treated at a high temperature of 100.degree. C. or higher. Therefore, the etch rate of the metal film becomes high, so that excellent etching without causing a partial etch residue in the metal film and a barrier film can be realized. The barrier film is a film inserted between the metal and polysilicon in order to suppress inter diffusion. As a metal, tungsten can be used. As a material of the barrier film, tungsten nitride can be used.
A conventional ordinary stage for an etching process having an electrostatic attracting means is formed by thermal spraying of Ni and Al onto a base aluminium alloy and, then, by thermal spraying of alumina ceramics to form a dielectric film for electrostatic attraction. A stage support also serving as a linear guide shaft and a linear guide bearing are so provided under the stage as to vertically move the stage when a wafer is loaded/unloaded onto/from the placement face.
When it is assumed that a vacuum treatment system having a conventional stage is applied to a wafer treatment at a high temperature, there are the following problems.
First, in the conventional stage using aluminium as a base material, the coefficient of thermal expansion of aluminium as a base material is 24.times.10.sup.-6 [.degree.C..sup.-1 ]. If the temperature is increased from room temperature to 160.degree. C., the thermal strain is .epsilon.=24.times.10.sup.-6 [.degree.C..sup.-1 ].times.(160-25) [.degree.C.]=3.2.times.10.sup.-3. At this time, a thermal strain is also introduced in the dielectric film for electrostatic attraction. The strain is obtained by .epsilon.1=7.times.10.sup.-6 [.degree.C..sup.-1 ].times.(160-25) [.degree.C.]=0.95 .times.10.sup.-3 from the coefficient of thermal expansion of alumina ceramics of 7.times.10.sup.-6 [.degree.C..sup.-1 ]. Since the Young's modulus of alumina ceramics is E=2.6.times.10.sup.4 [(kgf/mm.sup.2 ], the thermal stress applied on the dielectric film at this temperature difference is obtained by .sigma.=(.epsilon.-.epsilon.1).times.E=59 [kgf/mm.sup.2 ]. The stress is larger than 29 [kgf/mm.sup.2 ] of the strength of alumina ceramics. The alumina ceramics therefore become fractured and so electrostatic attraction cannot be employed.
Conventionally, the stage, the insulating plate, and the stage support are entirely in close contact with each other and have a structure such that heat escapes easily from the stage to the stage support. It is therefore difficult to maintain the wafer placement face of the stage at a high temperature. The tendency becomes more conspicuous as the amount of heat flow to the wafer increases and the diameter of the wafer becomes larger.
Further, conventionally, a linear guide bearing is positioned in the vacuum above a bellows. By using the stage support as a linear guide shaft, the stage is moved vertically without causing shaft runout. As the bearing, a solid bearing made of a fluororesin is employed. In the case of the conventional structure, when the stage is controlled at a high temperature, the temperature of the stage support serving as a guide increases due to the influence of the heat flow. In this case, there is a problem in that the clearance with the bearing becomes zero, the bearing functions as a brake, and the stage can not move vertically.